There exist a number of conventional solutions for BLDC motor control and processing of back electromagnetic field (EMF) signals generated by the motor. The back-EMF signals are signals generated by the rotor permanent magnet while it is spinning which have a force in the opposite direction of that in which the rotor is spinning. The back-EMF signal amplitude is proportional to the rotor rotation speed.
Regardless the control method, the conventional controller should provide a phase switching signal with some phase shift relative to the back-EMF signal. The purpose of the phase switch signal is to spin or drive the motor in the ‘forward’ direction. The phase shift between the phase switching signal and the back-EMF signal should be maintained at a constant value, regardless of the speed of operation of the motor. This phase shift can be formed by using the analog low-pass filters or using the digital signal processing methods. This invention is related to the use of analog processing techniques for forming the phase shifted control signals.
FIG. 1 shows a conventional motor system 100 having a rotor indicated by a rotating permanent magnet 110, and a first connections A 120, a second connection B 130, and a third connection C 140 providing 3-phase motor drive signals to the rotor. The A 120, B 130 and C 140 signals are motor winding connectors which are coupled to the phase voltage switch (not shown). The rotor is marked schematically as a two pole permanent magnet. The left column marks the rotor position at the start of a commutation and the right column indicates the rotor position at beginning of the next commutation. The coil drive voltage polarity is marked by a positive sign “+” and a negative sign “−”, and no-sign means that no drive voltage is applied to the coil at this phase.
For determining the rotor 110 position the non-powered coil is used and the inducted voltage from this coil is sensed. During motor operation sequence the drive signals is applied to the two motor terminals at the same time. The rotation of magnetic rotor 110 generates a back-EMF (electro-magnetic force) signal in the motor coils, the signal from the non-powered coil can be easily detected and processed because there is no power (drive) voltage applied to the coil at this time. This back-EMF signal can be used to determine when to switch the next phase by feeding it to a threshold comparator. The threshold comparator reference voltage is set to a value proportional to half of the supply voltage. During motor operation the phase commutation event is approximately 30 degrees delayed from the back-EMF voltage. This approximately 30 degree delay provides optimum rotor position when at new phase drive voltage applying momentum. This delay can be implemented using a tunable low-pass filter (LPF). This method can be called “classic” or conventional BLDC control method.
The rotation speed of rotor 110 can be adjusted by tuning the coil drive voltage. This can be accomplished by using a pulse width modulated (PWM) source for supply voltage modulation. Steps 150, 155, 160, 165, 170, 175 on FIG. 1 shows the rotation of rotor 100 and the signals on the three phases signals A 110, B 120, and C 130.
FIG. 2 shows a waveform plot 200 of phase angle 210 of the signals versus the table state of the output 220. A plurality of low pass filters (LPF) are used to generate the signals A 120, B 130 and C 140 for 30 degree phase shifting. The signals ‘A30’ 230, ‘B30’ 235 and ‘C30’ 240 are generated by passing the drive signals A, B, C through a low pass filters. These filters are used for forming the drive signals phase delay relative to the back-EMF signals. The symmetry of signals ‘A30’ 230, ‘B30’ 235 and ‘C30’ 240 depends on the pulse width modulation duty cycle because the comparators threshold value should be proportional to the half of the coil's effective (filtered PWM) power supply voltage. In the conventional solution the filtered back-EMF signal is compared with the fixed motor driver power supply voltage (not coils drive voltage). As a result, the filter output signals symmetry depends on coils drive signal duty cycle. For the optimal motor control the phase delay should depending on the of rotation speed only. In the conventional solution the phase delay depends on the pulse width modulation duty cycle that is undesirable for motor control.
A conventional phase switching signal generator circuit 300 is shown in FIG. 3. The conventional phase switching circuit 300 comprises a first sense input (Sense A) 310, a second sense input (Sense B) 312, and a third sense input (Sense C) 314. The sense inputs are coupled respectively to a first low pass filter 320, a second low pass filter 322, and a third low pass filter 324. A roll-off frequency 380 is coupled to each of the low-pass filters 320, 322, and 324. The roll-off frequency 380 is selected in such way to provide an approximately 30 degree phase shift at the selected rotation speed. This conventional solution does not take into account the filter frequency/gain characteristics, instead it sets the desired output signal phase shift.
The conventional phase switching circuit 300 further comprises a first comparator 330 having an input 332, an inverting input 334 and an output 336. The circuit 300 further comprises a second comparator 340 having an input 342, an inverting input 344 and an output 346. The circuit 300 further comprises a third comparator 350 having an input 352, an inverting input 354 and an output 356.
The filtered output 321 of first low pass filter 320 is coupled to input 332 of comparator 330, and to inverting input 354 of comparator 350. The filtered output 323 of first low pass filter 322 is coupled to input 342 of comparator 340, and to inverting input 334 of comparator 330. The filtered output 325 of first low pass filter 324 is coupled to input 352 of comparator 350, and to inverting input 344 of comparator 340.
The conventional phase switching circuit 300 further comprises a phase switching logic circuit 360. The first switched signal SW1 from output 336 of comparator 330 is coupled to the phase switching logic circuit 360. The second switched signal SW2 from output 346 of comparator 340 is coupled to the phase switching logic circuit 360. The third switched signal SW3 from output 356 of comparator 350 is coupled to the phase switching logic circuit 360.
A disadvantage of the conventional solution is that it requires three comparators and 3 tunable low-pass filters, which may require more resources that are available in some programmable devices.
It would be desirable to have a solution that uses a lower number of hardware resources.